Nonaqueous electrolyte batteries (for example, lithium ion batteries) having a high energy density attract attention as a large-sized and large-capacity power supply used for electric vehicles (EV), hybrid electric vehicles (HEV), electric motorcycle, and forklifts and the like. Development to make the lithium ion battery larger in size and to increase the capacity is under way while considering long life and safety. As the large-capacity power supply, battery modules including a large number of batteries connected with each other in series or in parallel to increase driving power have been developed.
The nonaqueous electrolyte battery includes a lithium ion battery using a carbon material for a negative electrode and a lithium battery using lithium titanate for a negative electrode.
The battery using the lithium titanate for the negative electrode has excellent life characteristics, safety, and input and output characteristics, and particularly excellent rapid-charge characteristics since a negative electrode potential is higher than that of a carbon negative electrode.
On the other hand, the nonaqueous electrolyte battery using lithium titanate for the negative electrode has a fast self-discharge speed and may have a high cost per capacity.
The nonaqueous electrolyte battery using lithium titanate for the negative electrode demonstrates rapid-charge ability when the nonaqueous electrolyte battery is charged using energy during braking and the like, such as at the time of the idling stop of an automobile, for example. However, the nonaqueous electrolyte battery is rapidly self-discharged and causes high cost per capacity as described above, which makes it difficult to practical use the nonaqueous electrolyte battery.
Examples of measures for compensating for this weak point include a method for connecting a nonaqueous electrolyte battery using lithium titanate for a negative electrode in parallel with a lead storage battery, the cost per unit capacity of which is low.
However, in this measure, there is disadvantage in that it is difficult to adjust the voltage of the nonaqueous electrolyte battery using lithium titanate for the negative electrode to the flat voltage of the lead storage battery.
Usually, a unit cell of a lead storage battery has a voltage of about 2.0 V. Generally, a commercially available lead storage battery is a 12 V or 24 V-battery in which unit cells, each of which is broadly available as a low-cost cell, are arranged in series. However, it is known that the lead storage battery has a flat operating voltage, and a state where the lead storage battery is held in a full charge state causes less battery deterioration. For example, when the 12 V-lead storage battery becomes the state in which the voltage is slightly more than 12 V which is the voltage at full charge, an electrolyte is decomposed, which causes a high probability of deterioration of the lead storage battery and generation of gas. When the lead storage battery becomes the state in which the voltage is slightly less than 12 V, the lead storage battery is in an overdischarge state, which promotes deterioration of the lead storage battery.
When a difference in voltages between terminals occurs in a plurality of batteries connected in parallel, the voltages between terminals are averaged so as to compensate the difference. Therefore, when a voltage between terminals in a battery connected in parallel with the lead storage battery and demonstrating rapid-charge-and-discharge characteristics and rapid-discharge characteristics, i.e., a nonaqueous electrolyte battery using lithium titanate for a negative electrode is largely changed during charge and discharge, the voltage of the lead storage battery is largely changed in accordance with the change, which may promote deterioration of the lead storage battery.